As adhesives for bonding printed circuit boards having formed circuit patterns on the surface thereof together in such a manner that said circuit patterns are opposite to each other, there are known, for example, sheet-like adhesives (connecting sheets) formed from adhesive compositions containing conductive particles dispersed in heat fusible and electrically insulating adhesive components (see Japanese Patent L-O-P Publns. Nos. 206772/1987, 40183/1987 and 40184/1987).
When the above-mentioned connecting sheet is interposed between circuit patterns of two pieces of printed circuit boards and the resulting assembly is pressed with heat, the insulating adhesive component of said connection sheet melts and move in the lateral direction of the circuit patterns overlapping each other and only the conductive particles are held between the circuit patterns, thus the portions of the overlapping circuit patterns having interposed the conductive particles between said portions can be connected electrically to each other and, at the same time, two pieces of the printed circuit boards can be bonded to each other by means of the insulating adhesive component of the connecting sheet.
As the conductive particles used in the connecting sheet as illustrated above, there have heretofore been used metallic particles, resin-coated metallic particles prepared by coating a metallic core material with a resin, and metal-coated resin particles having thereon a metallic layer formed by plating the surface of a resin core material with a metal.
However, when the connecting sheet containing as the conductive particles, for example, metallic particles, was used in the case as mentioned above, there was such a problem that circuit patterns contiguous to the circuit patterns to which said connecting sheet has been applied are apt to short-circuit when come in contact with said metallic particles. Further, there is also such a difficulty in manufacturing the connecting sheet containing metallic particles as the conductive particles that because of a large difference in gravity between the metallic particles and the insulating adhesive component, it is difficult to disperse said metallic particles in said insulating adhesive component. Furthermore, because such metallic particles as contained in the connecting sheet in most cases are not uniform in shape and particle diameter and also high in hardness, said metallic particles interposed between the circuit patterns will not be deformed even when a pressure is applied thereto and the contact area of said metallic particles with said circuit patterns becomes very small and, therefore, the use of the connecting sheet containing such metallic particles as mentioned above is apt to result in poor conduction of current in the connected terminal portions.
In order to solve such problems associated with the use of the metallic particles as mentioned above, there are used metallic particles having formed a resin coat layer on the surface of said metallic particles. The resin-coated metallic particles as mentioned above do not have conductibility when they are in a usual state, but the conductibility of said resin-coated metallic particles is exhibited when the resin coat layer of the resin-coated metallic particles held between the circuit patterns formed on two pieces of printed circuit boards is destroyed by the pressure applied to said printed circuit boards. Accordingly, the aforementioned problem that the contiguous circuit patterns that come in contact with the metallic particles are apt to short-circuit may be solved by the use of the connecting sheet containing such resin-coated metallic particles as mentioned above. Basically, however, the use of the metallic particles in the connecting sheet remains unchanged, and the aforementioned problem of poor conduction of current in the connected terminal portions of the circuit patterns due to nonuniformity of dispersion of the metallic particles in the insulating adhesive component and nonuniformity of particle diameter of the metallic particles still remains unsolved.
On the contrary, metal-coated resin particles having a metallic coat on a core material made of resin will have a relatively large contact surface, because they deform when pressed between the circuit patterns, hence the above-mentioned poor conduction of current in the connected terminal portions of the circuit patterns is hard to occur and, moreover, they have good dispersibility in the insulating adhesive component, because the core material of said particles is made of resin and there is no large differences in gravity between the particle and the insulating adhesive component. However, the surface of the metal-coated resin particles is composed of metal, hence the short circuit caused by contact of these metal-coated particle with the contiguous circuit patterns cannot be prevented.
As can be seen from the foregoing, it is hard to said that the conductive particles used in the conventional anisotropic conductive adhesives as mentioned previously have such characteristics sufficient for the purposes intended.
In the above-mentioned conventional anisotropic conductive adhesives, thermoplastic resins are used mainly and frequently as the insulating adhesive components in which such particles as mentioned above are to be dispersed.
The advantage that can be derived from the use of the thermoplastic resin as the insulating adhesive component in the anisotropic conductive adhesive as mentioned above is that the printed circuit boards having the circuit patterns on their surface can be bonded together as desired by heating and pressing at a relatively low temperature and for a relatively short period of time.
However, in the anisotropic conductive adhesive using the thermoplastic resin as the insulating adhesive component, the thermoplastic resin used as the insulating adhesive component has such a side that said resin cannot be said to have a sufficient stability to standing or aging, and particularly said insulating adhesive component is apt to have flowability when heat is applied thereto for a long period of time under the circumstances of high humidity. Accordingly, when the anisotropic conductive adhesive having such insulating adhesive component as above is used under the conditions as mentioned above, the conductive particles held between the circuit patterns sometimes move as the insulating adhesive component comes to flow, whereby conductibility between the circuit patterns, that is, a value of electrical resistance, becomes unstable.
In order to solve such problems as mentioned above, thermosetting resins are used also as the insulating adhesive components.
By virtue of the use of the thermosetting resins as the insulating adhesive components in the anisotropic conductive adhesive, stability to moisture and heat and reliability of said adhesive are sharply improved. The thermosetting resins, however, involve such problems that the resins are generally short in working life (time), and they require high temperature and extended period of time when used in application for bonding purposes.